1. Field of the Invention
The present invention relates to a control system and method for an automatic transmission. Particularly, the present invention resides in a control system and method for an automated MT (MT: Manual Transmission) which results from automating clutch/shift operations of a conventional manual transmission.
2. Description of Related Art
Manual transmissions popular in automobiles are small-sized, light-weight and highly efficient, in which a transmission is connected to an engine through a clutch. A system in which any of such transmissions and clutch are automated is generally called an automated MT (AMT: Automated Manual Transmission). This automated MT is expected as a next-generation transmission system capable of attaining both reduction of fuel consumption and easy drive.
In the above system, a shift operation for clamping a synchronizer sleeve to a free rotatable gear is automated and therefore a control method to be performed in the event of failure of the shift operation is important. In Japanese Patent Laid-Open Publication No. Hei 11 (1999)-082710 is described a method wherein, in case of a sleeve being unable to be clamped to an idle gear, a pushing load is once diminished and thereafter the sleeve is pushed against the idle gear with the previous load. According to this method, if the sleeve cannot be moved to a predetermined position, the pushing load is once diminished and the sleeve is moved again. Therefore, both sleeve and gear can be engaged with each other without increasing the load on a shift actuator mechanism and without re-starting the shift operation.
However, according to the control method described in the above laid-open publication, there is a fear that the operation for clamping the sleeve to the free rotatable gear may be delayed because the sleeve pushing load is once diminished. In the automatic transmission there is generally required a shift time of about 0.2 to 0.3 [s] and, for preventing a worsening of drivability, it is necessary to avoid a decrease of the pushing load as far as possible.
As shift operation failing cases there are a case where the sleeve is caught in a balk ring interposed between the sleeve and the free rotatable gear and a case where the sleeve is caught in a meshing part (hereinafter referred to as “gear dog”) of the free rotatable gear. More particularly, as examples of the former case there are; (1) a case where the synchronizing power of the balk ring is deficient due to a lack of the pushing load for the sleeve or the influence of disturbance (e.g., clutch drag torque or a change in load of a transmission output shaft) and
(2) a case where lubricating oil is not fed to between the balk ring and a cone face of the gear dog, resulting in sticking of the balk ring to the gear dog.
And as examples of the latter case there are;
(3) a case where there occurs differential revolution due to the influence of disturbance (e.g., clutch drag torque or a change in load of a transmission output shaft), making it impossible to clamp the sleeve to the free rotatable gear), and
(4) a case where deterioration of the balk ring causes the sleeve to stroke to the free rotatable gear before the rotation is synchronized. For preventing the worsening of drivability as far as possible it is necessary to perform a re-clamping operation such as increasing the sleeve pushing load to effect quick clamping of the sleeve in the case of (1) or once diminishing the sleeve pushing load in the case of (2) to (3). Further, in the case of (4), it is necessary to replace the balk ring with another one. However, in order to minimize breakage of the transmission in the event of limp home, it is desirable to inhibit the use of the free rotatable gear immediately.
Thus, it is necessary to prevent a worsening of drivability in the event of failure of a shift operation, thereby ensure clamping of the sleeve, and for minimizing breakage of the transmission it is necessary to perform a control which is applicable to plural events.